In a conventional hydrocarbon production installation, particularly offshore, the natural hydrocarbon reservoir is located in the subsoil. It consists of a volume of porous rock mainly comprising hydrocarbons in the gas and/or liquid state, and salt water. One or more wells are drilled to convey the fluids from the reservoir to the surface installations.
Hydrocarbon production is said to be flowing when the fluid pressure is sufficiently high within the reservoir to make the fluid rise naturally in the well and make the effluents reach the surface production units. However, in most cases, the flowing feature is absent, at least during part of the production period, particularly at the end of production. It is then necessary to artificially compress the fluids to make them rise to the surface and to operate at a requisite pressure.
In fact, conventional means for raising the pressure are only suitable for dealing with a single-phase fluid, that is, a gas or a liquid, but they are not suitable for dealing with a multiphase fluid, such as a petroleum effluent. Thus, pumps are known capable of raising the pressure of a gas-free liquid, and compressors are known capable of raising the pressure of a liquid-free gas.
In order to raise the pressure of a multiphase fluid of the petroleum effluent type, it is therefore necessary to separate the liquid and gas phases prior to their treatment, by a pump and a compressor respectively. Conventionally, the phases are separated using a tank or vessel, that is, a large volume unit in which the gas and liquid are separated by gravity. However, the operating pressure in a system of this type remains limited due to the large volume of a separation tank: this is because working at high pressure implies designing a tank with a very thick wall. This conventional system also has a number of drawbacks in terms of size and safety. It is particularly indispensable to provide safety depressurization means such as valves, vents or flares.
Other existing systems are installations called “WELLCOM” by CALTec which provide a compression of the hydrocarbon effluents issuing from low pressure wells using hydrocarbon effluents issuing from high pressure wells and achieve this in jet pumps or ejectors. A separation in a compact separator is provided in the case in which the effluents are multiphase, in order to compress the liquid with the liquid on the one hand, and, optionally, the gas with the gas on the other hand. If a high pressure well is lacking, the liquid portion can be compressed before serving in its turn to increase the pressure of the gas portion in a jet pump.
Document SPE 48934 (Carvalho et al., SPE Annual Technical Conference and Exhibition, September 1998) describes the combination of an electric submersible pump (ESP) and a jet pump in a hydrocarbon well. The ESP compresses the liquid hydrocarbons, and the gaseous hydrocarbons are entrained by the compressed liquid hydrocarbons using the jet pump.
Furthermore, document WO 2006/010765 describes a system comprising an “in line” separator upstream of distinct compressors for the gas, oil and water. The fluid residence time in the separator is short, so that this system is unsuitable for operation in slug flow conditions.
Another drawback of some of the abovementioned systems is associated with the mechanical transmission which is positioned on either side of the chamber walls, to apply forces to the fluids, said transmission raising a potential safety problem.
Besides these separate compression systems, other devices exist for raising the pressure of a multiphase fluid without separating the fluid phases. These include multiphase pumps. However, these devices remain complex and costly. This is because they require inlet fluid pretreatments to guarantee a minimum proportion of liquid, as well as cooling equipment, which accordingly demand safety equipment. They involve bulky, massive technologies, whose implementation entails a large scale design and manufacturing process. Their use also demands complex maintenance. They further often comprise rotating seals (mechanical seals), which are potential sources of gas leakage.
A need therefore exists for a method and a device for easy implementation thereof, for compressing a multiphase fluid to a high pressure, and which does not have the abovementioned drawbacks. In particular, a need exists to be able to adapt the capacity of the device to the evolution of the reservoir.